Your troops have to take the gateway, just by moving through it and no enemy troops being there I think. For sieges I usually stick to ladders/sap points myself, rams fall way too easily. Thats only for stone walls+ though, I use ballistas to take out the gates of the lesser defenses.

Has anyone had any huge siege battles yet? I had one where it was 2-3k egyptians versus my 1200 Town Watch. Battle lasts awhile with my men bravely trying to hold the wall. It was great to see that little 'fighting to the death' icon flashing. They got slaughtered to a man but held off the enemy from getting to the gate for awhile. Once the gateway fell my general charged out into the oncoming horde to die a glorious death. Sadly I only took about 600 of them with me in the end. Thats not bad for Town Watch versus Desert Axemen/Nile Spearmen though.